Switch mechanism for reversible grinder

ABSTRACT

A switch mechanism for use with reversible handheld power tools comprises a pair of switches that co-operate using abutment to restrict displacement of each switch according to the position of the other switch. In a tool having a dynamic brake and a series-wound motor, one of the switches has a false neutral position wherein a continuous current path is maintained between the tool motor and the dynamic brake when the second switch is moved from REVERSE or FORWARD to the neutral position.

TECHNICAL FIELD OF THE INVENTION

This invention relates to a switch mechanism for use with reversiblepower hand tools. More particularly, this invention relates to a switchmechanism for reversible power hand tools equipped with a dynamic brakeand a series-wound motor.

BACKGROUND OF THE INVENTION

Handheld power tools tend to be small, lightweight and portable, makingthem useful in many situations. However, these tools can also bedangerous, in that they may continue to operate even after the user hasturned it off. For example, momentum may cause the grinding wheel on areversible grinder to rotate even after the user has released thetrigger. To alleviate the problem, many power tools incorporate adynamic brake and provide a default position for the power switch thatengages the dynamic brake. Dynamic brakes typically short the motorwindings through a resistor. In order to increase the amount of brakingpower, it is known to sometimes employ an auxiliary set of windings inseries with the motor windings.

U.S. Pat. No. 5,600,107 to Tsai and U.S. Pat. No. 5,380,971 to Bitteldisclose trigger mechanisms for power tools with dynamic brakingsystems. In both patents, depressing or squeezing the trigger moves theelectrical contacts within the switch, completing a power circuit andoperating the tool's motor. The triggers are spring-biased to the OFFposition. Releasing the trigger causes the electrical contacts to moveback to the OFF position, both breaking the motor supply circuit andcompleting a separate dynamic braking circuit. Thus, two sets ofcontacts are required to move before the dynamic brake is activated andthe motor stops. Neither patent is applicable to reversible power tools.

A typical reversible power tool directional switch has FORWARD, OFF andREVERSE positions. In the OFF position, current flow through the motorwindings is interrupted. However, in order for a dynamic brake tooperate, a current path through the motor windings must be maintained.

U.S. Pat. No. 5,892,885 to Smith discloses a complicated dynamic brakingcircuit, for use with a reversible power tool. The dynamic brake isengaged by disconnecting the power source and speed control circuit,then connecting two other contacts, creating a low resistance pathbetween the motor terminals. The means for switching the direction ofoperation of the motor comprises a pivoting lever that is independent ofthe actuating trigger. It therefore appears that it is possible, usingthe switch disclosed by Smith, to switch the direction of tool operationbetween FORWARD and REVERSE without any intermediate state, therebypresenting a significant risk to the user.

U.S. Pat. No. 5,638,945 to Fukinuki discloses a safety mechanism in theform of a lock or blocking element associated with the trigger, so as tophysically prevent the trigger from moving out of position until theuser does so intentionally by releasing the lock. A sliding safetyswitch is disposed on opposite sides of a tool handle. The safety switchconsists of a generally T-shaped sliding lock button that isspring-biased to return to the locked position upon release by the user.An abutment surface extends from the trigger and physically blocks thesliding switch, preventing accidental depression of the trigger andoperation of the tool. Once the sliding switch is moved to anoperational position, it no longer blocks the trigger abutment surface.When the trigger is depressed, a hooked extension at the front of thetrigger engages an inversely hooked extension at the front of thesliding switch. This contact locks the tool in an operative mode bypreventing the switch from sliding back to a neutral position, even ifthe trigger is released. Depressing the trigger again unlocks the hook,allowing the tool to shut down when the trigger is released. While thehook mechanism reduces hand fatigue by allowing the user to operate thetool without maintaining pressure on the handle, if the useraccidentally drops the tool while the hook is engaged, the tool willcontinue to operate. This poses a significant danger to both the userand to any person or thing around him. Further, the switch mechanismdisclosed by Fukinuki is unsuitable for use in a power tool with adynamic brake, as the switch does not actively make or break anelectrical contact, as is required for a dynamic braking system.

It is an object of the present invention to provide a switch mechanismthat is suited for a reversible power tool having a dynamic brake. Theswitch arrangement according to the invention is designed to maintain acurrent path through the motor windings when a FORWARD, OFF and REVERSEswitch is moved to the OFF position. There is also provided a safetymechanism to ensure that the dynamic brake is engaged prior to switchingbetween the FORWARD and REVERSE positions.

The foregoing and other objects of the invention will be appreciated byreference to the summary of the invention and to the detaileddescription of the preferred embodiment that follow.

SUMMARY OF THE INVENTION

The switch mechanism according to one aspect of the invention includes aswitch having FORWARD, OFF and REVERSE positions. The switch mechanismis mounted on a reversible power tool having a motor, a trigger and adynamic brake. The trigger activates the motor when it is depressed andcompletes the dynamic brake circuit when it is released. When the switchis in the OFF position, the trigger is physically locked by the switchinto the released position, corresponding to the brake mode, such thataccidental pressure on the trigger cannot activate the motor. When theswitch is moved to the FORWARD or REVERSE position, the trigger may bedepressed to activate the motor, but the switch can not return to theOFF position until the trigger is released to engage the dynamic brake.

In the preferred embodiment, the locking mechanism between the switchand the trigger comprises an abutment between a portion of the switchand a portion of the trigger. When the switch is in the OFF position,the abutment prevents the trigger from being depressed. When the switchis moved to the FORWARD or REVERSE positions, the trigger may bedepressed but abutment is thereby established between the switch and thetrigger preventing movement of the switch until the trigger is released.This arrangement prevents the accidental movement of the switch to theOFF position while the trigger is depressed. It also prevents directtransiting between the FORWARD and the REVERSE positions.

The switch is biased to the OFF position by springs. Once the trigger isreleased, the switch automatically returns to the OFF position, lockingthe trigger in the released (brake) mode.

The switch is configured such that when it moves from the FORWARD orREVERSE position to the OFF position, its internal contacts maintain theFORWARD or REVERSE circuit path through the motor. This is done byestablishing a “false neutral” position within the switch correspondingto the OFF position. The current path required to operate the dynamicbrake is therefore maintained by the switch. As noted above, movement ofthe switch from the FORWARD or REVERSE position to the OFF position isalso constrained, by the abutment and bias arrangement, to necessarilycoincide with the release of the trigger to engage the dynamic brake.This ensures that the motor will not operate when the switch is in theOFF position, despite the switch maintaining a current path through themotor.

The false neutral position is created by a cavity within the switch thatis large enough to allow the switch to travel to the OFF position, butwithout yet switching the internal contacts. The switch must be fullydisplaced in the opposite operative position, in order to change theselection of internal contacts and to establish the circuit for tooloperation in the opposite direction.

In one aspect, the invention relates to a switch mechanism for use in apower tool having a motor and at least two operable directionscomprising a first switch having an ON and an OFF position toselectively actuate said motor; and a second switch to select betweensaid directions, said second switch further comprising a neutralposition in which neither of said directions is selected. A firstportion of said first switch is adapted to abut a first portion of saidsecond switch when said second switch is in the neutral position andsaid first switch is in said OFF position; said abutment of saidrespective first portions preventing displacement of said first switchto said ON position until said second switch is displaced to a positionother than said neutral position. Further, a second portion of saidfirst switch is adapted to abut a second portion of said second switchwhen said first switch is in said ON position and said second switch isin a position other than said neutral position, said abutment of saidrespective second portions preventing displacement of said second switchto said neutral position until said first switch is displaced to saidOFF position.

In a more specific aspect, the second switch of the invention comprisesan approximately inverted-U-shaped external sliding switch, with acavity on the uppermost underside of the external sliding switch and aninternal switch.

In a more specific aspect, the internal switch may be an approximatelyt-shaped sliding switch, or a toggle switch. The first switch may be atrigger.

In a further aspect, the switch mechanism of the invention may comprisemeans to bias the external sliding switch to a centred position, saidcentred position corresponding to the neutral second switch position.Such biasing means may comprise one or more springs. The springs may bemaintained in place by a connection to the external sliding switch. Theconnection may be made using one or more roll pins.

In yet a further aspect, the switch mechanism of the invention maycomprise a mounting mechanism to secure the switch mechanism in place,wherein said mounting mechanism comprises end blocks of a shape and sizeto snugly fit into the handle of the power tool and one or more pairs ofrods separating the end blocks.

In a further aspect, the switch mechanism of the invention may beinstalled in a power tool with a series-wound motor. Further, the powertool may also comprise a dynamic brake wherein said dynamic brake isengaged when said second switch is in said neutral position. The dynamicbrake may comprise auxiliary windings in series with said series-woundmotor.

In yet a further aspect, the switch mechanism may comprise at least onepair of motor contacts to allow operation of the tool in each of theoperable directions.

In another aspect, the second switch of the switch mechanism maycomprise an approximately inverted-U-shaped external sliding switch witha cavity on the uppermost underside of the external sliding switch, aninternal switch, electrical contacts extending from the lowermostsurface of the internal sliding switch, and one or more rocker contactsserving to connect the electrical contacts extending from the internalsliding switch to the motor contacts.

In a further aspect, an electric current path runs through said motor,said motor contacts, said rocker contacts and said electrical contactswhen said first switch is in said ON position and said second switch ispositioned to select one of said operable directions.

In yet a further aspect, an electric current path runs through saidmotor, said motor contacts, said rocker contacts, said electricalcontacts and said dynamic brake when said first switch is in said OFFposition and said second switch is in said neutral position.

In another aspect, the invention relates to a switch mechanism for usein a power tool with a motor and a dynamic brake and at least twooperable directions comprising a first switch having an ON and an OFFposition to selectively actuate said motor and a second switch to selectbetween said directions, said second switch further comprising a neutralposition in which said dynamic brake is engaged.

In a further aspect, the invention relates to a switch mechanism whereinsaid switches are constrained such that the first switch can move to theON position only when the second switch is not in the neutral position.The switches may also be constrained such that the second switch canmove to the neutral position only when the first switch is in the OFFposition.

In a more specific aspect, the switches may be constrained by physicalabutment of a portion of said first switch to a portion of said secondswitch.

In another aspect, the switch mechanism of the invention may comprise amounting mechanism to secure the switch mechanism in place, wherein saidmounting mechanism comprises end blocks of a shape and size to snuglyfit into the handle of the power tool and one or more pairs of rodsseparating the end blocks.

The foregoing was intended as a broad summary only and was not intendedto define the limits or requirements of the invention. Other aspects ofthe invention will be appreciated by reference to the detaileddescription of the preferred embodiment and to the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiment of the invention will be described by referenceto the drawings in which:

FIG. 1 is an isometric view of a reversible grinder with a portion ofthe grinder handle housing cut away to reveal the switch mechanism ofthe preferred embodiment;

FIG. 2 is a side view of the grinder and switch mechanism of FIG. 1;

FIG. 3 is a sectional view of the switch mechanism and grinder, taken online 3—3 of FIG. 2;

FIG. 4 is a sectional view of the switch mechanism and grinder triggerwith the trigger in an operational position and the switch in theFORWARD operational position, taken on line 4—4 of FIG. 3;

FIG. 5 is an isometric view of the switch mechanism;

FIG. 6 is a side view of the switch mechanism of FIG. 5;

FIG. 7 is a sectional view of the switch mechanism, taken on line 7—7 ofFIG. 6;

FIG. 8 is a top view of the switch mechanism of FIG. 5;

FIG. 9 is a sectional view of the switch mechanism, taken on line 9—9 ofFIG. 8;

FIG. 10 is an alternate embodiment of the internal switch mechanism ofFIG. 9; and,

FIG. 11 is a schematic of the circuit in the preferred embodiment of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

Referring to FIGS. 1 and 2, the switch mechanism 10 is shown in place inthe handle 12 of a reversible handheld power tool, such as a grinder 14,operated by a series-wound motor 51 (not shown in FIGS. 1 and 2).

A pivoting trigger 16 is disposed in the grinder handle 12 in oppositerelation to the switch mechanism 10. The positions of the trigger 16 andswitch 10 in the grinder handle 12 allow one-handed operation of thetrigger and switch mechanism, leaving the other hand free to control thegrinder via the hand grip 18. The power tool is provided with a dynamicbraking system comprising auxiliary windings 56 connected in series withthe windings of the grinder's series-wound motor 51, as best shown inFIG. 11.

The switch mechanism 10 generally comprises an internal switch (notshown in FIGS. 1 and 2) inside an external switch 20. Switch 20 hasFORWARD and REVERSE positions, as well as a central OFF position. Switch20 is spring-biased to the OFF position. In the OFF position, switch 20maintains a physical contact with the trigger 16, preventing depressionof the trigger 16. In this released position, internal contactsassociated with the trigger 16 switch from the power supply circuit tothe dynamic brake circuit, as best appreciated by reference to FIG. 11.

The external switch 20 portion of the switch mechanism 10 protrudesthrough a slot 22 in the grinder handle 12. External switch 20 is shownin the drawings as a square thumb slide 24, though the exact shape andsize of the external switch is unimportant. Thumb slide 24 may becovered with a larger knurled thumb pad (not shown) to allow a bettergrip on and hence more control over the external switch. A larger thumbpad could also completely cover the clearance slot 22, preventing dustand dirt from entering the grinder 14.

Referring now to FIGS. 1, 2 and 3, external switch 20 comprises a pairof opposed legs 26 depending from the thumb slide 24. Two safety dogs 28extend upwardly from the trigger 16 and are positioned to directlyunderlie legs 26 when switch 20 is in the OFF position. The legs 26 anddogs 28 therefore abut one another when an attempt is made to depresstrigger 16 while switch 20 is in the OFF position. This abutmentprevents depression of the trigger 16.

Two pairs of compression springs 30 bias the external switch 20 to theOFF position. If the user sets external switch 20 to FORWARD or REVERSE,but without depressing the trigger 16, the external switch 20 returns tothe OFF position, maintaining the dynamic brake circuit and preventingaccidental operation of the grinder.

When the user slides the thumb slide 24 out of the OFF position, thedepending legs 26 are displaced in relation to the dogs 28, allowingdepression of the trigger 16. When the user then squeezes trigger 16,the dogs 28 lift up to a position adjacent the depending legs 26, suchthat the sides of the legs 26 and dogs 28 abut, as shown in FIG. 4. Thisphysically impedes external switch 20 from returning to the OFFposition. Upon the release of trigger 16, dogs 28 are also withdrawndownward along with the trigger and hence out of side to side abutmentwith the legs 26. This allows the switch 20 to then be set to theFORWARD or REVERSE positions. The invention allows the user to releasethumb slide 24 once the trigger is depressed. This reduces hand fatigue,as compared to being required to maintain constant pressure on the thumbslide 24.

The external switch 20 according to the preferred embodiment issubstantially U-shaped, as best seen in FIG. 3. A switch contactcarriage 33 is provided between the depending legs 26, as will bediscussed in more detail below. The internal switch mechanism 32 will bediscussed in more detail below.

FIG. 5 shows a more detailed view of the assembly of the external switch20, including the thumb slide 24 and the depending legs 26. The entireswitch mechanism 10 is mounted between two end blocks 34. The exactshape and size of the end blocks 34 will depend on the dimensions of thehandle 12. End blocks 34 must fit snugly into the handle 12, such thatpressure on the thumb slide 24 will move only the thumb slide 24, notthe entire switch mechanism 10. Two pairs of drill rods 36, 38 attachedto the end blocks 34 with screws 40, maintain the end blocks 34 in aspaced relationship. The lowermost pair of drill rods 36 extends throughthe depending legs 26, providing a guideway along which the externalswitch 20 may slide. The uppermost pair of drill rods 38 supports thetwo pairs of compression springs 30, maintaining the springs 30 inproper alignment and position relative to the external switch 20. Thecompression springs 30 and upper drill rods 38 extend through theexternal switch 20. A roll pin 42 may be used to attach both of thespring 30 ends to the centre of each side of the external switch 20, asbest seen in FIGS. 6 through 8. The roll pins 42, or any comparablefastening mechanism, ensure the entire external switch 20 is centredproperly, such that the switch 20, in the absence of external pressure,is securely spring-biased to remain in the centre, OFF position.

FIG. 7 illustrates the attachment of roll pins 42, as well as the twopairs of drill rods 36, 38 extending through the depending legs 26. Inthe preferred embodiment, the external switch 20 must travelapproximately half an inch to operate the grinder 14 in the REVERSE orFORWARD directions. This is sufficient to ensure that the grinder 14will not operate unless the user intends to do so, and not if the useraccidentally bumps the handle 12 or thumb slide 24. The springs 30 areselected to allow easy movement of the thumb slide 24 by the user, whilestill providing a bias to the switch to the OFF position.

FIG. 7 shows switch carriage 33, containing two contacts 44 of theinternal switch 32 and two REVERSE contacts 46 for the grinder motor.FIG. 8 shows a top view of the switch mechanism, including the pair ofREVERSE motor contacts 46 and the ends of the pair of rocker contacts 45(only one of each of which is labelled in FIG. 8).

FIG. 9 shows the side view of one embodiment of the switch mechanism 10,including one contact 44 of the internal switch 32, one rocker contact45, one REVERSE direction contact 46 of the grinder motor and oneFORWARD direction contact 48 of the motor. For ease of explanation, theREVERSE direction contact 46 is specified as being on the left side ofFIG. 9, while the FORWARD direction contact 48 is on the right. Theinternal switch 32 slides from left to right, corresponding to theuser's thumb pressure on the thumb slide 24. The internal switchcontacts 44 thus connect with rocker contacts 45, pushing the ends ofrocker contacts 45 into contact with one set of motor contacts 50 at atime. When the internal switch contacts 44 are in contact with eitherset of motor contacts 50, a circuit through the series-wound grindermotor 51 is completed. This circuit remains unbroken, even if thetrigger 16 is released, until the user slides the thumb switch in theopposite direction. When the thumb slide 24 is displaced in either theFORWARD or REVERSE direction, and the trigger 16 is depressed, powerflows from the power source to operate the grinder 14 in the appropriatedirection.

The thumb slide 24 of the external switch 20 is also shown in FIG. 9,including an inner cavity 52 underneath the thumb slide 24. Thedimensions of the inner cavity 52 are determined such that the externalswitch 20, once released, will move back to its centre, OFF positionwithout moving the internal switch contacts 44 away from the motorcontacts 46, 48. In this position, the legs 26 depending from the thumbslide 24 abut the dogs 28 extending from the trigger 16, providing aninterlocking safety mechanism to ensure that trigger 16 can not beaccidentally depressed. Further, the circuit path established by theinternal switch 32 remains intact through the auxiliary windings 56 tooperate the dynamic braking system. Thus, the OFF position of theexternal switch 20 is actually a “false neutral.” This “false neutral”position makes this switch mechanism 10 particularly suitable forreversible power tools with dynamic braking systems.

An alternate embodiment of the internal switch contacts 44 is shown inFIG. 10. The external thumb slide 24 and depending legs 26 are the sameas those in FIG. 9, as are the directional motor contacts 48, 49 androcker contacts 45. However, internal switch 32 has been replaced bytoggle switch 54, which contains internal switch contacts 44. The toggleswitch arrangement provides the user with a more obvious indication ofwhen the operational direction of the switch changes, as the toggleswitch 54 snaps from one direction to the other. Using toggle switch 54may also provide a better seal against dirt entering the switchmechanism than the internal slide switch shown in FIG. 9. The externalthumb slide 24 still contains inner cavity 52 of sufficient size toallow a “false neutral” switch position.

A specific example of the “false neutral” arrangement is shown in FIG.9. In FIG. 9, the thumb slide 24 has returned to the apparent OFF orneutral position, but the internal switch contacts 44 remain connectedto the rightmost (FORWARD direction) motor contacts 48 via rockercontacts 45. If the user still wants to operate the grinder 14 in theFORWARD direction, he will slide the thumb slide 24 to the left,releasing the depending legs 26 from contact with the trigger dogs 28,then squeeze the trigger 16 (not shown). In order to change thedirection of operation of the grinder, the user would slide the thumbslide 24 to the extreme left. This would change the internal switchconnection such that the internal switch contacts 44 are connected, viarocker contacts 45, to the REVERSE direction motor contacts 46. When theuser squeezes the trigger 16, the grinder 14 would then operate in theREVERSE direction. When the thumb slide 24 is fully pressed in eitherdirection, the dogs 28 on the trigger 16 (not shown in FIG. 9) abut theside of the depending legs 26, such that the user no longer has to holdthe thumb slide 24 as long as the trigger 16 is depressed. As soon asthe user releases the trigger 16, the thumb slide 24 will return to theneutral position, the depending legs 26 will physically lock off thetrigger 16, and the dynamic brake will engage.

The switch mechanism described above ensures that current flow throughthe series-wound motor 51 windings is never interrupted unless the useris actively taking steps to change the direction of grinder operation.This arrangement guarantees that the dynamic braking circuit through theauxiliary windings 56 is always complete when the grinder is notactually being used. Further, the switch mechanism provides a secondsafety feature, namely an interlocking effect between the trigger 16 andthe switch 10 to prevent accidental depression of the switch. Thephysical interlock also allows the user to release the switch 10 oncethe grinder 14 is operating in the proper direction, reducing handfatigue.

It will be appreciated by those skilled in the art that other variationsto the preferred embodiment described herein may be practised withoutdeparting from the scope of the invention, such scope being properlydefined by the following claims.

1. A switch mechanism for use in a power tool having a motor, a handleand at least two operable directions comprising: a first switch movablebetween an ON position and an OFF position to selectively actuate saidmotor; a second switch to select between said directions, said secondswitch further comprising a neutral position in which neither of saiddirections is selected, said second switch being positioned across saidhandle in opposed relation to said first switch; a first abutmentextending from said first switch; a second abutment extending from saidsecond switch; said first and second abutments extending generallytoward one another in generally opposed, parallel relationship, saidsecond abutment being displaceable by movement of said second switchbetween a first position in which said first and second abutments are inend to end contact when said second switch is in said neutral position,and second and third positions in which said first and second abutmentsare in side to side contact when said second switch is in a positionother than said neutral position; said end to end contact preventingdisplacement of said first switch to said ON position until said secondswitch is displaced to a position other than said neutral position; and,said side to side contact preventing displacement of said second switchto said neutral position until said first switch is displaced to saidOFF position.
 2. The switch mechanism of claim 1 wherein said secondswitch comprises: an approximately inverted-U-shaped external slidingswitch; a cavity on the uppermost underside of the external slidingswitch; and an internal switch.
 3. The switch mechanism of claim 2wherein said internal switch is an approximately t-shaped slidingswitch.
 4. The switch mechanism of claim 2 wherein said internal switchis a toggle switch.
 5. The switch mechanism of claim 1 wherein saidfirst switch is a trigger.
 6. The switch mechanism of claim 3 or 4further comprising means to bias the external sliding switch to acentred position, said centred position corresponding to said neutralsecond switch position.
 7. The switch mechanism of claim 6 wherein saidmeans to bias the external sliding switch comprises one or more springs.8. The switch mechanism of claim 7 wherein said springs are maintainedin place by a connection to the external sliding switch.
 9. The switchmechanism of claim 8 wherein said connection is made using one or moreroll pins.
 10. The switch mechanism of claim 1 further comprising amounting mechanism to secure the switch mechanism in place, wherein saidmounting mechanism comprises: end blocks of a shape and size to snuglyfit into the handle of the power tool; and one or more pairs of rodsseparating the end blocks.
 11. The switch mechanism of claim 1 whereinsaid motor is a series-wound motor.
 12. The switch mechanism of claim 1further comprising a dynamic brake wherein said dynamic brake is engagedwhen said second switch is in said neutral position.
 13. The switchmechanism of claim 12 further comprising at least one pair of motorcontacts to allow operation of the tool in each of the operabledirections.
 14. The switch mechanism of claim 13 wherein said secondswitch comprises: an approximately inverted-U-shaped external slidingswitch; a cavity on the uppermost underside of the external slidingswitch; an internal switch; electrical contacts extending from thelowermost surface of the internal sliding switch; and one or more rockercontacts serving to connect the electrical contacts extending from theinternal sliding switch to the motor contacts.
 15. The switch mechanismof claim 14 wherein an electric current path runs through said motor,said motor contacts, said rocker contacts and said electrical contactswhen said first switch is in said ON position and said second switch ispositioned to select one of said operable directions.
 16. The switchmechanism of claim 14 wherein an electric current path runs through saidmotor, said motor contacts, said rocker contacts, said electricalcontacts and said dynamic brake when said first switch is in said OFFposition and said second switch is in said neutral position.
 17. Theswitch mechanism of claim 16 wherein said motor is a series-wound motor.18. The switch mechanism of claim 17 wherein said dynamic brakecomprises auxiliary windings in series with said series-wound motor. 19.The switch mechanism of claim 18 wherein said internal switch is anapproximately t-shaped sliding switch.
 20. The switch mechanism of claim18 wherein said internal switch is a toggle switch.
 21. The switchmechanism of claim 14 wherein said first switch is a trigger.
 22. Theswitch mechanism of claim 19 or 20 further comprising means to bias theexternal sliding switch to a centred position, said centred positioncorresponding to said neutral second switch position.
 23. The switchmechanism of claim 22 wherein said means to bias the external slidingswitch comprises one or more springs.
 24. The switch mechanism of claim23 wherein said springs are maintained in place by a connection to theexternal sliding switch.
 25. The switch mechanism of claim 24 whereinsaid connection is made using one or more roll pins.
 26. A switchmechanism for use in a power tool with a motor and a dynamic brake andat least two operable directions comprising: a first switch having an ONand an OFF position to selectively actuate said motor and a firstabutment; a second switch to select between said directions, said secondswitch further comprising a second abutment and having a neutralposition in which said dynamic brake is engaged; said switches beingconstrained such that said first switch can move to said ON positiononly when said second switch is not in said neutral position and saidsecond switch can move to said neutral position only when said firstswitch is in said OFF position; wherein said switches are constrained byphysical contact between said abutments.
 27. The switch mechanism ofclaim 26 further comprising at least one pair of motor contacts to allowoperation of the tool in each of the operable directions.
 28. The switchmechanism of claim 27 wherein said second switch comprises: anapproximately inverted-U-shaped external sliding switch; a cavity on theuppermost underside of the external sliding switch; an internal switch;electrical contacts extending from the lowermost surface of the internalsliding switch; and one or more rocker contacts serving to connect theelectrical contacts extending from the internal sliding switch to themotor contacts.
 29. The switch mechanism of claim 28 wherein an electriccurrent path runs though said motor, said motor contacts, said rockercontacts and said electrical contacts when said first switch is in saidON position and said second switch is positioned to select one of saidoperable directions.
 30. The switch mechanism of claim 29 wherein anelectric current path runs though said motor, said motor contacts, saidrocker contacts, said electrical contacts and said dynamic brake whensaid first switch is in said OFF position and said second switch is insaid neutral position.
 31. The switch mechanism of claim 30 wherein saidmotor is a series-wound motor.
 32. The switch mechanism of claim 31wherein said dynamic brake comprises auxiliary windings in series withsaid series-wound motor.
 33. The switch mechanism of claim 32 whereinsaid internal switch is an approximately t-shaped sliding switch. 34.The switch mechanism of claim 32 wherein said internal switch is atoggle switch.
 35. The switch mechanism of claim 32 wherein said firstswitch is a trigger.
 36. The switch mechanism of claim 33 or 34 furthercomprising means to bias the external sliding switch to a centredposition, said centred position corresponding to said neutral secondswitch position.
 37. The switch mechanism of claim 36 wherein said meansto bias the external sliding switch comprises one or more springs. 38.The switch mechanism of claim 37 wherein said springs are maintained inplace by a connection to the external sliding switch.
 39. The switchmechanism of claim 38 wherein said connection is made using one or moreroll pins.
 40. The switch mechanism of claim 26 further comprising amounting mechanism to secure the switch mechanism in place, wherein saidmounting mechanism comprises: end blocks of a shape and size to snuglyfit into the handle of the power tool; and one or more pairs of rodsseparating the end blocks.